As is well known, when a number of optical channels are propagating over an optical fiber, so-called stimulated Raman scattering (gain) may cause an optical channel to interact with a channel of a longer wavelength. Such interaction causes the power in the shorter wavelength channel to decrease and power in the longer wavelength channel to increase. In effect, the power in the shorter wavelength channels is “pumped” into the longer wavelength channels. The most pronounced effects occur when the channels are separated by about 15 THz. When an appreciable number of channels are transmitted over an optical fiber with a high level of power per channel, then the effect tilts the power divergence between the channels significantly to the channels of longer wavelengths.
The effect of Raman scattering increases when more than one band of optical channels are transported over an optical fiber, e.g., C and L bands. In that instance, the effect is approximately linear with channel separation, and may be determined by summing the contribution provided by each of the channels. If the different bands of channels are produced by different sources, then the possibility arises in which an entire band of channels may be suddenly lost or present based on whether the corresponding source has suddenly failed or come on line. This problem would be manifested by a sudden change in the spectra of the other bands, which may significantly increase the error rate of those bands. Consequently, the affected bands need to be adjusted immediately, e.g., within microseconds, to changes in average signal level and tilt.
The prior art uses an optical spectrum analyzer to generate the information needed to make the above adjustments.
However, what is needed is a sensor that quickly analyzes a band of channels to quickly detect changes in power level due to Raman scattering/pumping whenever the number of channels in another band of channels changes.